Green’s Dictionary of Slang

third degree v.

also put on the third degree
[third degree n.]

to interrogate forcefully and/or violently; thus third-degreeing n.

[Aus]Bulletin (Sydney) 2 July 36/3: It isn’t necessary to ‘third degree’ some of these suburbanites to find out what they are thinking about.
[US]J. Black You Can’t Win (2000) 207: I wasn’t taken out of my cell and ‘sweated’ or third-degreed, or beaten up.
[Aus]‘Banjo’ Paterson Shearer’s Colt 62: I expect they are going to put the third degree on his mate and see if he’ll squeal before they take Jimmy in.
[UK]C. Day Lewis Otterbury Incident 116: ‘We’ll put the Prune to the question’ [...] ‘Yeah. Third-degree him,’ said Nick.
[US]T. Runyon In For Life 192: He wasn’t gently paving the way for a little third-degreeing, was he?
[US] in S. Harris Hellhole 153: Cindy explains the point well in her story of her ‘third-degreeing’ by a certain officer.
[US]N. De Mille Smack Man (1991) 128: They’re really third-degreeing every pusher and junkie they get their hands on.